1. Field of the Invention
This invention relates to a control system for a compression-ignition internal combustion engine.
2. Description of the Related Art
Gasoline-fueled internal combustion engines known as “compression auto-ignition” or “compression-ignition” engines have emerged in various types in recent years. As taught by Japanese Laid-Open Patent Application No. 2000-87749, in certain operating regions these engines use compression-ignition, i.e., ignition in which the air-fuel mixture supplied to the combustion chamber auto-ignites and burns as a result of being compressed by the piston (sometimes called “Homogeneous Charge Compression-Ignition (HCCI)”), and in other operating regions they use spark-ignition, i.e., ignition in which the air-fuel mixture is ignited to burn by a spark plug. These engines can realize better thermal efficiency and fuel consumption performance than ordinary spark-ignition engines because they can achieve higher compression ratios.
The system described in the reference is directed to enabling constant stable operation of a compression-ignition engine by disabling compression-ignition operation when the engine coolant temperature detected by a coolant temperature sensor is lower than a predetermined temperature set as a function of the intake air temperature detected by a temperature sensor.
Since, however, compression-ignition operation is superior to the spark-ignition with regard to NOx reduction and thermal efficiency (fuel consumption performance), it is preferable to expand the compression-ignition operation utilization region.